GT250 only runs on choke

[Kenny11]

Hi all, I'm new to this forum, so have a 1977 gt250B and have just got it running but it will only run with the choke on, when I turn the choke off it just dies.
I've had the carbs cleaned and put new inlet rubbers on
Any ideas would be gratefully accepted.
Thanks in advance
Al

[GTandcbr]

Have you checked and adjusted the fuel/float levels in the carbs? The choke or enrichment circuit draws fuel from the bottom of the float bowl so when you shut it off if the fuel level is too low the pilot circuit cannot draw any petrol into the venturi. Try adjusting the fuel level see if that solves it.
Also just check the airscrews are turned out the correct amount

[Evans Ward]

Also, do you know for sure that the internal brass jets in your carbs are genuine Mikuni? If you used jets from a kit, that could be suspect. Make sure the jets are the correct size AND Mikuni. It matters too whether you are using the stock airbox or individual pods with jetting.

[Kenny11]

Thanks for the replys, yes the floats were set to the recomended setting and the air screws are turned out 1 1/4 turns, I haven't put new jets in the carbs, they were removed and all cleaned and put back, so I'm at a loss

[GTandcbr]

Try running the bike with the airbox off next so we can eradicate that as a problem. Remove the plugs check to see are they wet or dry? When you push the choke in does the bike suddenly stop or get slower and slower? Another suggestion is remove the pilot/idle jets and look on the side there shouls be a number stamped on, are they the correct size?

[Kenny11]

Ok, I'll try the air box thing this morning and let you know, if I can get to the garage without drowning

[Kenny11]

So I removed the air filter and fired her up, but could only run it for a minute or so as it filled the garage up with smoke but it's still dies when the choke is pulled up. Possibly not warm enough I'll have to wait till the rain stops to do it outside

[GTandcbr]

So its worth noting that, unlike four stroke engines, 2 strokes dont like to idle off choke when cold. The 2 stroke needs heavy enrichment to start untill the internal components become warm enough to vaporise the fuel oil mixture. Once you do go outside when its dry let the bike warm up and rev it before shutting off the choke. It is important to shut the choke off once its warmed up though
Just had an afterthought. Put the petcock on prime position in case its a fuel supply issue.(but dont forget to put it back after)

[Kenny11]

So it's stopped raining at last, I made sure it was good and warm turned off the choke and spent a bit of time adjusting the tick over screws and it now ticks over ok
Still doesn't sound crisp really, but it is chucking out a fair bit of oil from the right exhaust, and when I tried to pull away there is no power, it bogs down.

[GTandcbr]

So progress. There will be all kinds of crap in the crankcase which will clear eventually when its had a good run. Make sure the plugs are clean and gapped they may be bunged up after all your problems trying to start it. Is the petrol new? Only takes a month for it to go off.

[Kenny11]

Yes, fuel is new, I'm going to remove the exhaust and drain out the oil that is in the silencer as there is a puddle on the garage floor this morning. Then try and find out if it's the oil pump or it's pulling oil through the crank seal.
Oh the joys of tinkering with old bikes

[GTandcbr]

The puddle of oil is more likely to be a mixture of carbon from the inside of the exhaust and water caused by consensation building up within the pipe. This is normally ejected when the bike is running normally. That why there are little drain holes at the bottom of the silencer.
I'm pretty sure from the symptoms you describe that the pilot jet or pilot jet passage and bypass are obstructed in some way. You could try turning the airscrews in the 3/4 of turn out to see if that inproves the throttle response
The best way to clean the internal passages of the pilot circuit is with an ultrasonic cleaner but if you dont have access to one simple carb cleaner sprayed in and blown out with compressed air may do the job

[Kenny11]

So, took it up the road yesterday, And it's running better but a new problem has turned up,
With Just a small twist on the throttle the revs go right up to the red line so I have to flick the off button, and restart it.
It is not the cable jamming so I'm thinking it must be a carb problem

[jabcb]

Some years ago had a throttle cable with a broken wire strand that caused one of the carb slides to not return properly. The damaged cable was between the 1-to-3 cable splitter & a carb. Your wrist raises the slide & springs lowers the slide. Doesn’t take much to overpower those springs.

Hanging slides is also a problem with flange mounted carbs. Over-tightened carb mounts can warp the carb to the point the slide sticks.
73-75 GT250s have flange mounted carbs & 76-77 GT250s have spigot mounted carbs. This doesn’t apply if you have a stock 77 model. But it could if you actually have an earlier model or have early carbs on your 77 model.

[Kenny11]

Some years ago had a throttle cable with a broken wire strand that caused one of the carb slides to not return properly. The damaged cable was between the 1-to-3 cable splitter & a carb. Your wrist raises the slide & springs lowers the slide. Doesn’t take much to overpower those springs.

Hanging slides is also a problem with flange mounted carbs. Over-tightened carb mounts can warp the carb to the point the slide sticks.
73-75 GT250s have flange mounted carbs & 76-77 GT250s have spigot mounted carbs. This doesn’t apply if you have a stock 77 model. But it could if you actually have an earlier model or have early carbs on your 77 model.

The cable is good, the sliders both return ok. Carbs are rubber mounted 77s, would a sticky float do this?